Lowermost Emsian to lower Givetian ostracods from the Western

Joannea Geol. Paläont. 11: 53-54 (2011)
Lowermost Emsian to lower Givetian ostracods from the Western Dra
Valley (Morocco): First palaeoecological and event-stratigraphical
data
Claudia DOJEN, Sarah ABOUSSALAM & Thomas BECKER
The Devonian of the Anti-Atlas in southwest Morocco is world-famous for its extensive
outcrops that are rich in well-preserved faunas, but those of the Western Dra Valley are
still under study and especially the lower Devonian ostracods are poorly known. We
present first palaeoecological and event-stratigraphical results on ostracods of earliest
Emsian to basal Givetian age from the sections Bou Tserfine, Rich Tamelougou and
Hassi Mouf near Assa, which are well dated by conodonts. All ostracod associations
indicate an offshore position below wave base. Palaeobiogeographically, most taxa belong the Palaeotethysian Province, but few North-American taxa are also present and
corroborate migration paths between both areas via North Africa.
At present, some hundred ostracod individuals belonging to about 45 benthic taxa
have been tentatively identified. Lower Emsian samples with ostracods are from sections Rich Tamelougou and Bou Tserfine. They yielded taxa such as Polyzygia kroemmelbeini and Bollia azagora, which are in these sections restricted to the Early Emsian although their global range is longer. But most of the ostracods are from the lower
Upper Emsian Hollardops Limestone (basal Khebchia Fm) and the Eifelian Pinacites
Limestone (lower Yeraifa Fm).
The Hollardops Limestone is a regionally distinctive limestone that has furnished
frequently Caudicriodus culicellus–Icriodus corniger ancestralis conodont assemblages, which indicate early Late Emsian age. Only its basal bed is latest early Emsian
as indicated by monospecific occurrences of the bilatericrescens conodont assemblage.
The ostracod assemblages belong to the so-called “mixed faunas” with only few palaeocopes but rich in metacopes (some of them with small spines), indicating an offshore
position probably in deeper and less agitated water below wave base. The Pinacites
Limestone contains goniatites (e.g., Pinacites, Fidelites), and conodonts of the costatus Zone (Eifelian). The ostracod fauna is partly similar to those of the Hollardops Limestone and long ranging taxa such as Ulrichia ex gr. acricula and Jenningsina planocostata occur throughout the sections. But several taxa as e.g., Bufina aff.
bicornuta, Bufanchiste bufinoides, Polyzygia symmetrica or Favulella frankenfeldi
are despite their global ranges restricted here to the Eifelian.
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Besides these, several spiny taxa such as Semibolbina, Loquitzella, Berounella
or Tricornina occur, thus, reflecting slightly deeper and calmer water conditions than
during the sedimentation of the Hollardops Limestone. So far, Givetian strata with ostracods have been found in the studied sections only at Hassi Mouf. The occurring taxa
are long-ranging and globally widely distributed such as Jenningsina planocostata or
Praepilatina, and the assemblages present low diversities.
As regards Devonian events and their effect on ostracods both the KellwasserEvent and the Hangenberg-Event are studied thoroughly. However, the smaller-scale
events such as the Daleje, Choteč or Kačák Event have not been studied in detail so far.
Our study gives preliminary information on the influence of these events on benthic ostracods faunas in the W Dra Valley:
— At the Daleje-Event (approx. Early/Late Emsian boundary), eight of thirteen taxa survive; Polyzygia vinea disappears worldwide, whereas P. kroemmelbeini disappears locally.
— At the Chotec-Event (approx. Emsian/Eifelian boundary), thirteen of twenty-four taxa
survive; Jenningsina thuringica disappears worldwide; Bufina sotoi and Tricornina ex gr. sp. A survive only in Morocco.
— At the Kajak-Event (approx. Eifelian/Givetian) only five from twenty-seven taxa survive; the five survivors are ubiquitous taxa, such as the long-ranging and globally
widely distributed Jenningsina planocostata; even some long ranging taxa such
as Ulrichia ex gr. fragilis, U. ex gr. spinifera and “Cytherellina” inconstans disappear locally.
Thus, the Kajak-Event seems to be the most effective one of the minor events as regards the ostracods of the Western Dra Valley.
Further studies have to include more material covering the event horizons in more
detail. In addition, the results have to be compared with the studies on Devonian ostracods from other Moroccan and North African areas as described e.g., by CASIER or by
BECKER.
Authors addresses:
Claudia Dojen
Landesmuseum Kärnten, Museumsgasse 2, A-9021 Klagenfurt, Austria
[email protected]
Sarah Aboussalam & Thomas Becker
Westfälische Wilhelms-University, Institute for Geology and Palaeontology, Corrensstrasse 24, D-48149 Muenster, Germany
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